1. Field of the Invention
The present invention generally relates to a variable phase shifting circuit and a variable phase shifting method. More specifically, the present invention is directed to such a variable phase shifting method, and also a variable phase shifting circuit easily manufactured in an integrated circuit with a simple circuit arrangement.
2. Description of the Related Art
Conventionally, RC variable phase shifting circuits play an important role specifically in signal processing systems operable in microwave frequency ranges. The RC variable phase shifting circuits are generally composed of resistor elements and capacitor elements, and any one of resistor element and capacitor element is constructed as a variable element.
Japanese Laid Open Patent Application (JP-A-Heisei 1-268217) describes the variable phase shifting circuit capable of realizing the broadband variable width. In this variable phase shifting circuit, the emitter of the first transistor is connected via the first resistor element to the first power supply terminal, and the collector of this transistor is connected via the second resistor element to the second power supply terminal.
The series circuit is connected between the collector of the first transistor and the emitter thereof. This series circuit is composed of the second transistor and the third resistor element. The base of this second transistor is connected to the emitter thereof as the variable capacitor element. Otherwise, the base of the second transistor is connected to the collector thereof.
In this variable phase shifting circuit, the emitter of the third transistor is connected via the fourth resistor element to the first power supply terminal, and the collector of this third transistor is connected via the fifth resistor element to the second power supply terminal.
The series circuit is connected between the collector of the third transistor and the emitter thereof. This series circuit is composed of the fourth transistor and the sixth resistor element. The base of this second transistor is connected to the emitter thereof as the variable capacitor element. Otherwise, the base of the third transistor is connected to the collector thereof.
Furthermore, in this variable phase shifting circuit, both the base of the first transistor and the base of the third transistor are connected to the input terminal. The junction point between the second transistor and the third resistor element is connected to the first output terminal. Also, the junction point between the fourth transistor and the sixth resistor element is connected to the second output terminal.
Either the emitter of the first transistor or the collector thereof is connected to the collector of the second transistor, and either the emitter of the third transistor or the collector thereof is connected to the fourth transistor. It should be noted that in this variable phase shifting circuit, a field-effect transistor (FET) may be employed as each of the above-explained transistors.
On the other hand, Japanese Laid Open Utility Application (JP-U-Heisei 2-126431) discloses a monolithic variable phase shifting circuit. In this reference, an input signal is supplied to a first phase shifting circuit and a second phase shifting circuit such that the input signal is separated into two signals having the phase difference of 90 degrees.
Also, in this monolithic variable phase shifting circuit, the first input signal is supplied to the first phase shifting circuit while the second input signal is supplied to the second phase shifting circuit, such that these input signals are synthesized into a signal not to have a phase difference.
In each of the first phase shifting circuit and the second phase shifting circuit, both of the collector of the transistor and the emitter thereof are grounded in order that the DC voltage can be applied via the resistor element. Also, the series circuit is connected between the collector of the transistor and the emitter thereof. This series circuit is composed of the variable capacitor element, the fixed capacitor element, and the resistor element. This variable capacitor element is formed by the junction capacitance of the transistor.
The base of the transistor is connected to the input terminal. Either the junction point between the variable capacitor element and the resistor element or another junction between the fixed capacitor element and the resistor element is connected to the output terminal.
Furthermore, since the variable capacitance element is connected In parallel to the fixed capacitor element, the degradation of the frequency characteristic can be suppressed. It should also be noted that field-effect transistors may be employed as the respective transistors in this monolithic phase shifting circuit.
Further, Japanese Laid Open Patent Application (JP-A-Heisei 3-26015) discloses the voltage variable phase shifter for controlling the phase of the signal by controlling the voltage by the transistor circuit.
In this voltage-controlled variable phase shifter, the collector of the first transistor is grounded so as to vary the phase of the input signal. The second transistor controls the emitter current supplied from the first transistor in response to the controlling variable voltage. Also, the fixed capacitor element is connected between the emitter of the first transistor and the ground.
In response to the change in the controlling variable voltage, this voltage-controlled variable phase shifter changes the output impedance of the first transistor. Furthermore, both this changed output impedance and the fixed capacitance element determines the phase shift amount.
FIG. 1A and FIG. 1B represent RC variable phase shifting circuits known in this technical field. The RC variable phase shifting circuit shown in FIG. 1A is composed of the resistor element R and the variable capacitor element C'. The predetermined phase shifting amount with respect to the input signal may be obtained by adjusting the capacitor value of the variable capacitor element C'.
Also, the RC variable phase shifting circuit shown in FIG. 1B is composed of the capacitor element C and the variable resistor element R'. The desirable phase shifting amount with respect to the input signal may be obtained by adjusting the resistor value of the variable resistor element R'.
In the case that the above-explained RC variable phase shifting circuits shown in FIG. 1A and FIG. 1B are realized as the integrated circuit, the following problem will arise. That is, it is practically difficult to assemble the variable resistor element R' into the semiconductor element as to the known RC variable phase shifting circuit shown in FIG. 1B.
Also, in the RC variable phase shifting circuit represented in FIG. 1A, for example, the variable capacitor realized by utilizing the reverse biasing of the transistor, which may function as the variable capacitor element C', has been proposed.
However, when the variable capacitor element by using the reverse biasing of the transistor is assembled into the semiconductor element, if the junction capacitor between the base and the emitter is utilized, then the voltage withstanding level is low.
Under such a circumstance, this reverse biasing variable capacitor element can be hardly applied to the semiconductor element. Also, when the junction capacitor established between the base-to-emitter path is used, the change amount of the phase shift is very small. As a consequence, it is practically difficult to obtain a desirable phase shift amount.